The present invention relates to analog processing, and more particularly to integrated circuits which perform analog processing functions.
Typically, analog integrated circuits are designed to receive one or more analog input signals, and process them by performing specific functions such as gain, attenuation, filtering, integration, addition and subtraction. These functions usually dictate the topology of the analog integrated circuit. For example, the topologic arrangement of operational amplifiers and resistors are adjusted to provide either inverting or non-inverting gain. Every topology has specific noise, distortion and offset voltage sensitivities. Changing an analog circuit's function necessitates a change in the topology of the analog circuit, which thus changes the noise, distortion and offset voltage characteristics of the circuit.
It is known to change a circuit's topology by reconnecting circuits at the board level or the integrated circuit level via wires, metal layers, etc. For example mask programmable circuits, gate arrays and linear circuit arrays allow the circuit and the function of the circuit to be changed at time of manufacture. However, even changes at this level cause changes in the parasitics of resistance and capacitance and thus lead to variations in performance.
When metal oxide semiconductor (MOS) switches are used to provide programmability by varying the topology, further parasitics are introduced. In particular, MOS transistors exhibit a square-law characteristic of gate to source voltage versus output current and thus MOS transistors can seriously increase distortion when they are placed in a signal path.